Arrange UV maps of multiple objects without joining them
There is a tool that will allow you to visualize the islands from other objects that are still separate.  To use it, select all the objects and enter Edit mode for the primary selection that you are currently working with and check View->Draw Other Objects.  Now all UV Islands for the secondary selections should be visible with a faint outline in the UV Image Editor.



If you would like seams for objects that are already unwrapped: Enter edit mode for the object and select everything; there is a tool available UVs->Seams From Islands that will add seams to all the islands.